The Applicant notes that after transplantation stem cell (SC) renewal is very limited as indicated by the reduced concentration of SC for second and third retransplantations. It is hypothesized that this is either due to prior treatment which may damage the marrow stroma or to the disruption of the marrow with harvest. The aim of the proposal is therefore to study self-renewal of SC under these conditions and alternate conditions. He will work with two separate populations of SC, rhodamine-123 (Rh-123low), a dull-stain population that is non cycling but capable of mediating long-term reconstitution and Rh-123hi cells, a brightly staining population which mediates short-term reconstitution of hematopoietic cells. Studies done in vitro have already shown that the Rh-123low cells can differentiate into Rh-123hi cells.